A tire testing apparatus disclosed in Patent Reference 1 is known conventionally. This tire testing apparatus measures uniformity (uniformity) of a tire at the time of rotating the tire with the tire attached to a rim. This tire testing apparatus includes a pneumatic circuit 510 having a test system 520 for regulating an internal pressure of a tire 550 at the time of measuring the uniformity as shown in FIG. 5. The test system 520 of this pneumatic circuit 510 has air supply piping 521, a controller 523, and a pressure regulator (pressure proportional control valve) 525.
The air supply piping 521 makes connection between the tire 550 and an air source 530 such as a compressor, and guides compressed air from the air source 530 to the tire 550. The controller 523 outputs an internal pressure (target command pressure) of the tire 550 used as a target in the case of measuring the uniformity to the pressure regulator 525. The pressure regulator 525 is formed on the air supply piping 521, and regulates a flow rate of air of the inside of the air supply piping 521 so that the internal pressure of the tire 550 becomes the target command pressure. Concretely, the pressure regulator 525 maintains the internal pressure of the tire 550 at the target command pressure by supplying and discharging the air from the air source 530 to and from the tire 550 so as to become a flow rate proportional to a differential pressure (hereinafter simply called a “differential pressure”) between the internal pressure of the tire 550 and the target command pressure from the controller 523.
In the tire testing apparatus 500 described above, in a state in which the test system 520 of the pneumatic circuit 510 maintains the internal pressure of the tire 550 at the target command pressure, a drum (not shown) is pressed on the tire 550 and the tire 550 is rotated and a repulsion force generated in the tire 550 is measured by, for example, a load measuring device (not shown) formed on the drum and thereby, the uniformity of the tire 550 is measured.